During operation of a gas turbine, the gas turbine rotor blades of e.g. the turbine hot section of the gas turbine are exposed to elevated temperature gases and high rotational velocities. While gas turbine rotor blade tips may be coated as part of the manufacturing process, the tips may be “ground in the rotor” to ensure all the gas turbine rotor blades are the correct height and contoured properly. However during the grinding action, the protective coating is removed and environmentally sensitive base alloy of the gas turbine rotor blades is revealed. With thousands of subsequent hours of operation, the tips of the gas turbine rotor blades will oxidize, causing the gas turbine rotor blades to shorten, and allow for hot gases to escape past the tips instead of being captured by the airfoil for work. The result is a less efficient gas turbine.
The performance of gas turbines can be improved my by minimizing clearances between the tips of the gas turbine rotor blades and a stationary shroud or a stationary casing of the gas turbine. In order to maintain the requisite tight tolerances at the gas turbine rotor blade tips, an abrasive coating is applied to the rotor blade tips to preferentially cut into the shroud or the casing of the gas turbine. Cold tolerances between the shroud or casing and the rotor blade tip are designed such that as the rotor blade heats and expands, it contacts the shroud or the casing. During this contact, the rotor blades remove material from the shroud or the casing ensuring the clearance is minimal.
The abrasive coatings comprise abrasive particles embedded in a metal matrix. The present invention relates to a method for manufacturing an abrasive coating on a gas turbine component, especially on a gas turbine rotor blade tip.
Several process to manufacture an abrasive coating on a gas turbine component, especially on a gas turbine rotor blade tip, are known from the prior art.
U.S. Pat. No. 5,359,770 discloses a method for bonding abrasive blade tips to the tip of a rotor blade. This prior art discloses that abrasive blade tips may be applied as a separate step during manufacture, where an abrasive blade tip is brazed to the rotor blade tip at a maximum temperature of 1190° C., the blade tip having been manufactured with a cobalt-based boron containing alloy, and a boron containing braze. The rotor blade is heated uniformly to the processing temperature. For that, high temperatures may not be employed, since the consolidation temperature must be maintained below the temperature at which the base metal properties will be altered. Due to the concentrations of melting point depressants, namely boron, as well as the processing temperature a re-melting temperature of approximately 1200° C. may be expected.
U.S. Pat. No. 6,355,086 discloses a method on how to use direct laser processing to apply an abrasive blade tip to a gas turbine rotor blade post manufacture without having to subject the blade to potentially harmful temperature excursions. Due to the melting and re-solidification of the pre-alloyed powder, the material will show coring or a segregated microstructure.
According to U.S. Pat. No. 6,194,086 low pressure plasma spraying and according to U.S. Pat. No. 6,706,319 cold spraying have also been used in the past as a means to apply a metal matrix ceramic composite to tips of gas turbine rotor blades.